The method of pore volume determination is essential to the characterization of my invention. Mercury penetration techniques indicate that there is substantially no pore volume above 200A in diameter in the aluminas of my invention. However nitrogen absorption techniques indicate that while some pore volume in aluminas prepared by my invention exists above 100A, the pore volume in this range can be substantially reduced over a comparable alumina not treated in accordance with this invention. The difference between mercury penetration techniques and nitrogen absorption is a well-known phenomenon and thought to occur as a result of the pore configuration. Pores which are cylindrical in shape will give similar nitrogen absorption and nitrogen desorption isotherms. Irregularly shaped pores or "ink bottle" shaped pores will have significantly different pore volume distributions depending on the method of determination. For "ink bottle" shaped pores or pores having narrow entrances, the nitrogen desorption and mercury intrusion pore volume distributions are thought to be representative of the size of the pore opening or pore orifice. However, nitrogen absorption is not influenced by the pore opening but rather reflects the average diameter profile of the entire pore. This difference is important in characterizing catalysts since the size of the pore opening or orifice will act as a screen to the size of molecule which can diffuse into the catalyst. The alumina made by the use of my invention gives pore volume distributions which are typical of those thought to be caused by restricted or narrow necked openings to pores.
Thus, the novel alumina described in this invention provided an improvement in the art in that it allows molecules of a selective size to pass through an orifice into an enlarged pore cavity where maximum surface area is available for reaction.